Recently, from a viewpoint of securing a stable energy resource, the development of oil wells or natural gas fields having a high depth in a severely corrosive environment has been in progress. To realize such development, oil country tubular goods and a line pipe for transportation are strongly required excellent SSC resistance in a sour environment a yield strength YS of 125 ksi or more.
To satisfy such a demand, for example, in Patent Literature 1, there has been proposed a method of manufacturing steel for an oil country tubular goods where low alloy steel containing, by weight %, 0.2 to 0.35% C, 0.2 to 0.7% Cr, 0.1 to 0.5% Mo, 0.1 to 0.3% V, and further containing C, Cr, Mo and V in an adjusted manner is quenched at an Ac3 transformation temperature or above and, thereafter, is tempered at a temperature of 650° C. or above and an Ac1 transformation temperature or below. With the use of a technique described in Patent Literature 1, the composition of the steel for an oil country tubular goods can be adjusted such that a total amount of precipitated carbide is 2 to 5 weight %, a rate of MC type carbide among a total amount of carbide becomes 8 to 40 weight % thereby producing a steel for an oil country tubular goods having excellent sulfide stress corrosion cracking resistance.
In Patent Literature 2, there has been proposed a method of manufacturing steel for an oil country tubular goods having excellent toughness and sulfide stress corrosion cracking resistance where low alloy steel containing, by mass %, 0.15 to 0.3% C, 0.2 to 1.5% Cr, 0.1 to 1% Mo, 0.05 to 0.3% V and 0.003 to 0.1% Nb is processed by hot working being finished at 1000° C. or above after the low alloy steel is heated to 1150° C. or above, subsequently is quenched from a temperature of 900° C. or above and, thereafter, is tempered at 550° C. or above and Ac1 transformation temperature or below and, further, quenching and tempering treatment where the low alloy steel is reheated to a temperature of 850 to 1000° C., is quenched, and is tempered at 650° C. or above and an Ac1 transformation temperature or below is performed at least one time. With the use of the technique described in Patent Literature 2, the composition of the steel for an oil country tubular goods can be adjusted such that a total amount of precipitated carbide is 1.5 to 4 mass %, and a rate of MC type carbide out of a total carbide amount is 5 to 45 mass %, and a rate of M23C6 type carbide is 200/t (t: wall thickness (mm)) mass % or below thus manufacturing steel for an oil country tubular goods having excellent toughness and excellent sulfide stress corrosion cracking resistance.
In Patent Literature 3, there has been proposed a steel material for an oil country tubular goods containing, by mass %, 0.15 to 0.30% C, 0.05 to 1.0% Si, 0.10 to 1.0% Mn, 0.1 to 1.5% Cr, 0.1 to 1.0% Mo, 0.003 to 0.08% Al, 0.008% or less N, 0.0005 to 0.010% B, 0.008% or less Ca+O, and further containing one kind or two kinds or more of elements selected from a group consisting of 0.005 to 0.05% Ti, 0.05% or less Nb, 0.05% or less Zr, and 0.30% or less V, wherein a maximum length of a continuous non-metal inclusion by cross-sectional observation is 80 μm or less, and the number of non-metal inclusions having a grain size of 20 μm or more by cross-sectional observation is 10 pieces/100 mm2 or less. With the use of such a steel material for an oil country tubular goods, it is said that a low alloy steel material for an oil country tubular goods having high strength required for an oil country tubular goods use and having excellent SSC resistance which corresponds to such a strength can be acquired.
In Patent Literature 4, there has been proposed a low alloy steel for oil country tubular goods having excellent sulfide stress corrosion cracking resistance containing, by mass %, 0.20 to 0.35% C, 0.05 to 0.5% Si, 0.05 to 0.6% Mn, 0.025% or less P, 0.01% or less S, 0.005 to 0.100% Al, 0.8 to 3.0% Mo, 0.05 to 0.25% V, 0.0001 to 0.005% B, 0.01% or less N, and 0.01% or less O, wherein the relationship of 12V+1−Mo≥0 is satisfied. In the technique described in Patent Literature 4, in addition to the above-mentioned composition, the low alloy steel for oil country tubular goods may further contain 0.6% or less Cr to the extent that the relationship of Mo−(Cr+Mn)≥0 is satisfied, and the low alloy steel for oil country tubular goods may further contain one kind or more of elements selected from a group consisting of 0.1% or less Nb, 0.1% or less Ti, 0.1% or less Zr. The low alloy steel for oil country tubular goods may further contain 0.01% or less Ca.